1. Field of the Invention
The present invention relates to radio base station equipment and mobile station equipment for determining the locations of mobile stations in a mobile communication system.
2. Description of the Related Art
In recent years, because of liberalization of the market and competition among communication service providers, telephone charges and the prices of terminal equipments have been lowered. Furthermore, portable terminal equipments having various functions adapted for users' needs have been put into practical use. Consequently, the use of mobile communication systems has become rapidly widespread.
According to the aforementioned competition and the development of techniques adapted for multimedia communications, mobile communication systems are required to have higher added values. For example, it is strongly required that determination services be offered to a terminal equipment possessed by a person who encounters an emergent situation, to determine the position of the terminal equipment economically and accurately.
A known technique (hereinafter referred to as the first prior art technique) for accomplishing such determination using PHS is to cause a radio base station to discern terminals located in a wireless zone formed by the local station, by determining the position of a desired terminal by a wireless zone. In PHS, each wireless zone is generally formed as a microcell having a radius of one hundred meter to hundreds of meters.
Another known technique (hereinafter referred to as the second prior art technique) is available that can be applied to portable and in-vehicle terminal equipments. For example, such a terminal equipment is equipped with a receiver for receiving signals arriving simultaneously from four or three or more GPS (global positioning system) satellites. Radio determination is accomplished by obtaining root(s) of 4 or 3 determination equations using different information contents contained in the signals above. The information contents are three-dimensional locations of the individual GPS satellites and the relative distances to these satellites, that are obtained according to the information.
With the aforementioned first prior art technique, the wireless zone(s) that is/are the current location of a terminal which is the object of determination (hereinafter referred to as the determination target station) and is/are not always singular, depending on the zone configuration or on the establishment form of radio base stations.
Where the determination target station is located in a area where plural radio base stations are formed closely, it may be impossible to find that the determination target station is located in the wireless zone formed by the closest radio base station, due to overreach or multipath or for other cause. Hence, there is the possibility that the accuracy of determination deteriorates uselessly.
Furthermore, the radius of a wireless zone is set to a value adapted for the traffic distribution during the busy hour. Therefore, the radius is set to smaller values in urban areas. Conversely, the radius is set to larger values with increasing distance from urban areas.
Accordingly, the first prior art technique provides determination accuracy that is not always sufficiently high. In addition, determination target stations located in areas where there are no radio base stations cannot become a subject of determination.
With respect to the second prior art technique, in determination target stations located in urban areas, reception of signals transmitted by GPS satellite is hindered by tall buildings and other objects. Otherwise, the signals are received through multipath. Consequently, stable determination is not accomplished. Even when the determination is realized, the accuracy is not always sufficiently high.